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Figure 29.13 Fluorescent Antibody Stain of Pneumocystis carinii The yellow circles are P. carinii cysts.

Pathogenesis

The small spores of P. carinii, measuring only 1 to 3 mm, are easily inhaled into lung tissue. In experimental infections, the spores attach to the alveolar walls, and the alveoli fill with fluid, mononuclear cells, and P. carinii cells in various stages of development. Later, the alveolar walls become thickened and scarred, preventing the free passage of oxygen.

Epidemiology

Various strains of P. carinii are widespread among animals, including dogs, cats, horses, and rodents, persisting in their lungs as a latent infection. Although identical in appearance to strains infecting humans, those examined have been genetically distinct. Serological tests indicate that most children are infected by age two and a half. The infection is asymptomatic and is generally eliminated within a year. The source and transmission of human infections are unknown. Most cases of pneumocystosis occur in persons with immunodeficiency, but it is uncertain whether their disease is caused by activation oflatent infection, or infection newly acquired from inhalation of airborne spores. Epidemics among hospitalized malnourished infants and elderly nursing home residents suggest airborne spread, and P. carinii has been detected in indoor and outdoor air by using the polymerase chain reaction.

Prevention and Treatment

Pneumocystosis used to occur in about four-fifths of AIDS patients and was the leading cause of death. The disease is now largely prevented by starting regular doses of a medication such as trimethoprim-sulfamethoxazole for HIV disease as soon as the CD4+ T-cell count falls below 200 per ml, or if other hall-marks of immunodeficiency appear, such as thrush, a Candida infection of the mouth and throat.

29.3 Infectious Complications of Acquired Immunodeficiency 755

Trimethoprim-sulfamethoxazole is also among the best medications for treating pneumocystosis and, along with oxygen and other measures, can reduce mortality from nearly 100% to about 30%. Alternative medications are available for treating those who cannot tolerate trimethoprim-sulfamethoxazole because of its side effects—mainly rash, nausea, and fever. For unknown reasons, people with HIV disease are more likely to develop these side effects than others. After treatment for pneumocystosis, individuals with HIV disease must receive preventive medication indefinitely, or until they have a sustained rise in CD4+ T-cell count to above 200 cells/ml.

The main features of pneumocystosis are presented in table

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